The invention relates to a method and apparatus for correcting DC offset problems found in high-speed transceiver systems.
Conventional transceiver systems commonly must switch back and forth between transmit and receive modes. As the speed of data transmission increases, the time allowed for switching between the Transmit (Tx) and Receive (Rx) modes becomes smaller. One conventional type of receiver is known as a Direct Conversion zero intermediate frequency (IF) receiver. In this type of receiver a local oscillator is tuned to the carrier frequency of the incoming signal. These types of receivers commonly have multiple stages in which the incoming signal is down converted and processed using a local oscillator (LO) circuit. These IF type receivers create in phase (I) and 90 degrees out of phase quadrature (Q) signals from the received signal. Large DC offsets are produced in the down converter outputs of Zero IF receivers due to LO leakage at the RF ports of the down converters. Additional DC offsets exist along the I and Q paths that include low-pass channel filters and automatic gain control (AGC) circuits with large gains. Therefore each of these DC coupled stages introduces a DC offset error into the signal. In an orthogonal frequency division multiplexing (OFDM) system, a difference between the local oscillator (LO) frequency and the incoming signal frequency causes DC offset errors within the system to profoundly degrade the SNR after demodulation. These unwanted DC errors may also cause the amplifiers used in the I and Q branches to saturate. Once the amplifier is in a saturated state, the received data signal cannot be processed and amplified correctly so the received data signal is lost.
Prior art attempts to deal with the above problems have been only semi-successful. Stroet et al.'s article entitled “A Zero-IF Single Chip Transceiver for up to 22 Mb/s QPSK 802.11b Wireless LAN” shows that these offset errors may be reduced or settled in 25 microseconds. The reduced DC offset is still too high for OFDM systems and takes too long to settle. As mentioned above, with an increase in data speeds, this prior art system is not useable in today's transceiver environments.
All known prior art techniques used to reduce these DC offset errors have drawbacks in one form or another. For example, AC coupling signals with high frequency cut off values may reduce response time, but has an unacceptable signal to noise ratio or an unacceptable effect on the signal itself. Further, automatic gain control is also only useable when the DC offset level is very small. The end result is that these DC offset values can not be reduced in an acceptable amount of time.